This invention relates in general to electrophotography and, in particular, to electrophotoconductive imaging members having multiple layers.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotograhic plate to a support such as a paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors.
During machine function, a photoconductive imaging member is constantly under repetitive electrophotographic cycling which subjects the electrically operative layers to high electrical charging/discharging cycles, multiple exposures to light for latent imaging development and erasure, and heat due to temperature elevation as a result of machine operation. The repetitive electrical and light fatigue lead to a gradual deterioration in the electrical characteristics of the imaging member, and limit its service life in the field. In the attempt to fabricate a robust photoconductive imaging system, many innovative ideas have been attempted with the intent to overcome this shortfall and extend the electrical functional life of the imaging member.
Modern composite imaging members have been developed having numerous layers which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl layer and an optional overcoating layer.
One problem associated with multilayer electrophotographic imaging members is delamination. Since the various layers of a multilayer electrophotographic imaging member contain differing materials, the adhesion of those materials will vary. In particular, it is desirable to provide an adhesive layer between the charge blocking layer and the charge generating layer since adequate adhesion is not possible when certain materials are used for these layers. In particular, a charge generating layer comprising a polycarbonate, a polyvinylcarbazole, a polyvinylbutyral, or a polystyrene-butadiene copolymer binder does not adhere well to a silane charge blocking layer. Thus, the adhesive layer is provided to improve adhesion. However, although adhesion is improved with the use of an adhesive layer, the adhesive layer is not always adequate in providing the necessary adhesion.
A number of materials have been provided for the adhesive layer. For example, copolyesters such as du Pont 49,000 resin available from E.I. du Pont de Nemours & Company and Vitel PE-100 resin available from Goodyear Rubber and Tire Company, are commonly employed. With such copolyesters, adhesion may be increased in proportion with the thickness of the layer. However, the adhesion no longer increases once a particular thickness is obtained. Thus, although these copolyesters provide improved adhesion, the adhesion strength is still limited by the thickness of the adhesive layer, and there is always a need to further enhance this adhesion for mechanically robust imaging member performance.
The above-described copolyester resins have also been utilized as binders in the charge generating layer. For example, U.S. Pat. No. 4,415,639 to Horgan discloses in Example VI a photoconductive layer consisting of du Pont 49,000 polyester and vanadyl phthalocyanine coated upon an adhesive layer of du Pont 49,000 adhesive. However, the use of a material such as du Pont 49,000 as a binder in the charge generating layer has a number of problems. For example, the du Pont 49,000 resin binder does not adequately disperse photogenerating pigment, in particular trigonal selenium pigment, to warrant proper electrophotographic function.
The provision of an adhesive layer between the charge blocking layer and the charge generating layer is required, though undesirable since it involves an additional coating step in the fabrication of an electrophotographic imaging member. The additional coating step not only increases the time required to fabricate an imaging member, it also increases costs due to the requirement of additional materials, labor, and a decrease in production throughput. Since the need for the adhesive layer is not for assisting electrophotographic function but is for the mechanical purpose of linking the charge blocking layer to the charge generating layer, it would be economical if an electrophotographic imaging member could be fabricated without the requirement of the adhesive layer.
In electrophotographic imaging member fabrication, unfortunately, an adhesive layer is always required in connection with some charge generating layer compositions utilized with particular charge blocking layers. For example, polyvinylcarbazole is one of very few binders known to disperse selenium and selenium alloys which are commonly used in forming photogenerating layers. Polyvinylcarbazole does not adhere very well to certain charge blocking layers, especially charge blocking layers containing silanes. Since there are practically no alternatives other than using polyvinylcarbazole, polyvinylbutyral, polystyrene or polystyrene-butadiene copolymer to disperse the selenium, the adhesive layer has been absolutely required. Even then, adhesion of the charge generating layer is not always adequate to guarantee mechanical integrity during function.
U.S. Pat. No. 4,853,308 to Ong et al discloses a layered photoresponsive imaging member comprised of a supporting substrate, a photogenerating layer, and a hole transport layer comprised of bis(-diarylamino) fluorenes. The charge generating layer, as disclosed in Example X, may comprise a squaraine compound dispersed in Vitel PE-200 polyester from Goodyear.
U.S. Pat. No. 4,576,981 to Hilger et al discloses an adhesive composition comprising a copolyester containing at least one copolymer of vinylidene chloride, preferably in a weight ratio of about 3:1 to 1:1. The adhesive composition is disclosed as being suitable for use particularly on a polyester film.
U.S. Pat. No. 3,887,369 to Matsuno et al discloses an organic photoconductive element with an interlayer and adhesion promoting additive. In particular, a barrier layer comprised mainly of copolymers comprising alkyl vinyl ethers and maleic anhydride or of composites of alkyl vinyl ethers/alkyl half esters of maleic acid copolymers and polyvinyl pyrrolidone or copolymers thereof, is provided between a substrate and an organic photoconductive layer. The photoconductive layer adhesion is improved by incorporating vinylidene chloride/acrylic ester copolymers into the organic photoconductive layer.
U.S. Pat. No. 4,391,888 to Chang et al discloses multilayered organic photoconductive elements comprising a layer of polycarbonate resin which is capable of functioning as a barrier layer to prevent charge leakage, and as a bonding layer.
While the above-mentioned electrophotographic imaging members provide a number of alternatives for preparing electrophotographic imaging devices, there continues to be a need for multilayer electrophotographic imaging members which resist delamination. In particular, there continues to be a need for electrophotographic imaging members comprising a photoconductive layer comprised of selenium or selenium alloys which is capable of adhering to a charge blocking layer, especially a charge blocking layer formed from silanes.